Method of and device for fractionating suspensions

ABSTRACT

A device for fractionating pulp suspensions has a series of vertical filter sheets attached to a horizontal shaft. Suspension is sprayed against the sheets to effect a separation of coarse and fine solids. Cleansing liquid is also sprayed against the sheets to keep the filter holes open and the fabric sheets are movable with respect to the sprays.

The present invention relates to a method of fractionating suspensions,preferably containing fibre pulp, the suspension to be fractionatedbeing sprayed in the form of at least one jet against one of the sidesof a filter sheet during displacement of the filter sheet and said jetrelative to one another, so that the suspension is separated into a finefraction, which passes through the screen holes of the filter sheet andwhich contains fine particles, and a coarse fraction, which does notpass through said screen holes and which contains coarse particles andsome fine particles. The invention also relates to a device for such afractionation of suspensions.

In a known method of this kind the filter sheet is circular and is keptfixed in a horizontal position, the suspension being sprayedsubstantially vertically towards the upper side of the filter sheetduring rotation of the jet of the suspension in horizontal direction.The resulting relative displacement between the jet and the filter sheetaims at cleansing the screen holes of the filter sheet, so that theseare freed from initial clogging. Such a cleansing effect occurs, but hasproved workable only over a limited period of time.

When fractionating it is particularly important that the screen holesare kept free from initial clogging, since these negatively affect thefractionation result. This is so because the sizes of the particles inthe coarse and fine fractions are radically changed even with a moderateclogging of the screen holes, so that the intended fractionation cannotbe obtained. Therefore, when utilizing the known fractionation methodthe operation must be stopped regularly for cleansing the screen holesof the filter sheet.

The object of the present invention is to provide a new fractionationmethod of this general kind, which, during operation efficiently keepsthe screen holes clean from initial clogging, whereby stopping tocleanse the screen holes is avoided.

A further object of the present invention is to provide a newfractionation device for accomplishing the new fractionation method.

These objects are obtained by means of a method of the kind initiallymentioned, which is characterized primarily by spraying cleansing liquidin the form of at least one jet against the filter sheet, and displacingthe filter sheet and said jet of cleansing liquid relative to eachother, such that the jet of cleansing liquid hits the screen holes ofthe filter sheet which have been sprayed upon with suspension.

In this way an advantage is obtained in that the cleansing liquid can beallowed to spray against the filter sheet with a considerably strongerimpact pressure than the suspension, since in contrast to the suspensionthe cleansing liquid is free from relatively coarse particles, whichcould be pressed into the screen holes of the filter sheet and clog themif the suspension were sprayed at too high a pressure against the filtersheet.

The cleansing liquid is suitably sprayed on the same side of the filtersheet as the suspension if it is primarily required that the liquidconcentration of the coarse fraction be kept low. In this case theliquid content of the fine fraction is increased. In the case where theliquid concentration of the coarse fraction is of secondary significancethe cleansing liquid is advantageously sprayed on the opposite side ofthe filter sheet from the suspension, whereby the cleansing effect ofthe screen holes is further improved.

The cleansing liquid may be sprayed continuously or intermittentlyagainst the filter sheet. Intermittent spraying results in a lowerdilution of the coarse and fine fractions compared to continuousspraying.

Advantageously, the pressure is kept higher on the coarse fraction sideof the filter sheet than on the fine fraction side, whereby thefractionation capacity is increased and the liquid content of the coarsefraction is reduced.

Preferably, the filter sheet is kept substantially vertical and isdisplaced relative to the jets of suspension and cleansing liquid. Inthis way, the advantage is obtained that the coarse particles depositedon the filter sheet during the fractionation are continuously removedfrom the filter sheet with the coarse fraction, when the coarse fractionunder the influence of gravity flows off the filter sheet. Therefore,the jets of cleansing liquid can be arranged such that they only hit thescreen holes which are not covered by coarse particles, whereby suchcoarse particles do not run the risk of being pressed by the cleansingliquid into the screen holes of the filter sheet, clogging them.

The invention also relates to a new device for fractionatingsuspensions, preferably containing fibre pulp, comprising a housing, atleast one filter sheet with screen holes located in the housing, spraymeans for spraying suspension on one side of each filter sheet, meansfor displacing each filter sheet and the spray means relative to eachother, the suspension being separated during operation into a finefraction, which passes through the screen holes of each filter sheet andwhich contains fine particles, and a coarse fraction, which does notpass through said screen holes and which contains coarse particles andsome fine particles, and outlets in the housing for coarse fraction andfine fraction, respectively. The new fractionation device is primarilycharacterized in that further spray means are arranged to spraycleansing liquid against each filter sheet for cleansing the screenholes of the sheet, and in that said displacement means are adapted todisplace each filter sheet and said further spray means relative to eachother, such that during operation the further spray means spraycleansing liquid against the screen holes which have been sprayed withsuspension.

Either may each filter sheet be stationary while the spray means aredisplaceable or vice versa. However, it is preferred that each filtersheet is displaceable while the spray means are stationary, each filtersheet being arranged substantially vertically, which enables the designof a compact and simple fractionation device of high capacity.

Preferably, a substantially horizontal shaft extends into the housingand is rotatably journalled in the housing, each filter sheet beingattached to the shaft substantially transversely to it.

Suitably, at least one pair of filter sheets are arranged on the shaft,the spray means for suspension being arranged to spray the outer sidesof the filter sheets of each pair of filter sheets.

A plurality of embodiments of the fractionation device according to theinvention are possible. In the following paragraphs the invention willbe explained more exactly by a description of a number of embodiments,with reference to the accompanying drawings, in which:

FIG. 1 shows a longitudinal sectional view through a preferredembodiment of the fractionation device according to the invention,

FIG. 2 is longitudinal sectional view of an alternative embodiment ofthe invention in which a drive motor is replaced by hydraulic drivemeans;

FIG. 3 shows a sectional view along the line III--III of FIGS. 1 and 2,

FIG. 4 shows a cassette with a screen cloth for a filter sheet in thedevice according to FIGS. 1 and 2,

FIGS. 5 and 6 show sections along the lines V--V and VI--VI,respectively, of FIG. 4,

FIG. 7 shows spray means for suspension according to a section along theline VII--VII in FIGS. 1 and 2,

FIG. 8 shows a sectional view along the line VIII--VIII of FIG. 2,

FIGS. 9 and 10 show sectional views along the lines IX--IX and X--X,respectively, of FIG. 3,

FIG. 11 shows a longitudinal sectional view through another embodimentof the fractionation device according to the invention,

FIG. 12 shows a sectional view along the line XII--XII of FIG. 11,

FIG. 13 shows in detail a sealing arrangement of the device according toFIG. 11,

FIG. 14 shows a longitudinal sectional view through two fractionationdevices according to FIG. 11, which are assembled to a compactfractionation plant,

FIG. 15 shows a section along the line XV--XV of FIG. 14,

FIG. 16 shows a longitudinal sectional view through four fractionationdevices according to FIG. 11, which are assembled to a compactfractionation plant.

FIG. 17 shows a further embodiment of the fractionation device accordingto the invention,

FIG. 18 shows a section along the line XVIII--XVIII of FIG. 17,

FIG. 19 shows a section along the line XIX--XIX of FIG. 18,

FIGS. 20 and 21 show a modification of the fractionation deviceaccording to FIG. 11,

FIG. 22 shows a section along the lines XXII--XXII of FIGS. 20 and 21,

FIG. 23 shows an arrangement of spray means for suspension for afractionation device according to the invention,

FIG. 24 shows a section along the line XXIV--XXIV of FIG. 23,

FIG. 25 shows a modified spray means for suspension of the arrangementaccording to FIG. 23,

FIG. 26 shows a section along the line XXVI--XXVI of FIG. 25,

FIG. 27 shows a modified arrangement of a screen cloth for a filtersheet according to the invention,

FIG. 28 shows a part of the arrangement according to FIG. 27,

FIGS. 29 and 30 show sections along the lines XXIX--XXIX and XXX--XXX,respectively, of FIG. 25,

FIG. 31 shows a filter disc framework, on which the screen clothaccording to FIG. 27 is intended to be mounted, and

FIGS. 32 and 33 show sectional views along the lines XXXII--XXXII andXXXIII--XXXIII, respectively, of FIG. 31 with a mounted screen cloth.

The fractionation device shown in FIG. 1 comprises a housing 1, in whichtwo pairs of filter discs 2, 3 and 4, 5, respectively, are attached to ahorizontal shaft 6 transversely to and coaxially with the shaft. Theshaft 6 is rotatably journalled in the housing 1 and coupled to a drivemotor 7. A plurality of spray means 8 for suspension to be fractionatedis arranged to spray the outer sides of the filter discs of the twopairs of filter discs 2, 3 and 4, 5, respectively. At each filter disc anumber of the spray means 8 are arranged on a radially outer circularpipe 9 and a radially inner circular pipe 10 (FIG. 6), which circularpipes 9, 10 are situated coaxially with the filter discs. The pipes 9,10 at each filter disc are connected via an assembly pipe 11 to astationary distribution pipe 12 extending parallel to the shaft 6 andradially outside the filter discs 2-5. Each spray means 8 is providedwith at least one spray nozzle 8a, which is directed towards the screeenopenings of the filter disc situated closest to the spray nozzle. Thespray means 8 which are situated between the filter discs 3 and 4 areprovided with double opposite directed spray nozzles 8a.

In a bottom space in the housing 1, there are partition walls 13extending from the housing to the filter discs 2-5. In said bottom spacethe partition walls 13 form drainage chambers 14 for fine fraction.These chambers are situated between two adjacent partition walls 13 ateach pair of filter discs 2, 3 and 4, 5. Drainage chambers 15 for coarsefraction are situated axially outside said adjacent partition walls 13.The drainage chambers 14 for fine fraction are connected via bottomoutlets 16 to a drain pipe 17 and the drainage chambers 15 for coarsefraction are connected to a drain pipe 19 via bottom outlets 18.

A stationary distribution pipe 20 for cleansing liquid extends parallelto the shaft 6 and above the filter discs 2-5. From the distributionpipe 20, pipes 21 extend vertically down between the filter discs 2 and3, and 4 and 5, respectively. The pipes 21 are provided with stationaryspray means 22, which are directed towards the filter discs.Alternatively, the pipes 21 may be arranged to oscillate as indicated inFIG. 3.

Each filter disc 2-5 comprises four screen members in the form ofexchangeable cassettes 23 (FIG. 4). Each cassette 23 consists of ascreen cloth 24 and a frame 25, which extends through tubular channelsformed by the screen cloth 24 (FIGS. 5 and 6) and keeps the screen clothstretched. In each filter disc the cassettes 23 are attached to anannular peripheral bar 26 and four spokes 26a by means of thread joints(shown in FIGS. 9 and 10). Alternatively, each filter disc may bedesigned with more or less cassettes than four depending on the diameterof the filter discs.

During operation the filter discs 2-5 are rotated by the drive motor 7at the same time as suspension is continuously supplied to the spraymeans 8 via the distribution pipe 12, the assembly pipes 11 and thepipes 9 and 10, so that the rotating filter discs 2-5 are sprayed withsuspension. In this manner, the suspension is separated into a finefraction, which passes through the screen holes of the screen cloths 24of the filter discs and which contains fine particles, and a coarsefraction, which does not pass through the screen holes and whichcontains coarse particles and some fine particles. The coarse fractionflows down along the filter discs to the drainage chambers 15 and flowsfurther through the bottom outlets 18 to the drain pipe 19. The finefraction is collected in the drainage chambers 14 and flows therefromvia the bottom outlets 16 to the drain pipe 17. Cleansing liquid,preferably water, is supplied to the spray means 22 continuously orintermittently via the distribution pipe 20 and the pipe 21 and issprayed under a relatively high pressure against the screen cloth in adirection towards the fine fraction sides of the filter discs. In thismanner the screen holes are regularly cleansed from initial cloggings.

In an alternative arrangement, shown in FIGS. 2 and 8, the drive motor 7may be replaced by an arrangement of buckets 27. As shown in FIGS. 2 and8, the buckets 27 are arranged at the fine fraction side of at least onefilter disc, are evenly distributed along the circumference of said onefilter disc, and are oriented, so that during operation the bucketswhich move downwards will receive and keep fine fraction, while thebuckets which move upwards will be emptied of fine fraction. In a filterdisc provided with such buckets 27, the spray means 22 for cleansingliquid should be arranged on the coarse fraction side of the filterdisc, as shown at the filter disc 5 in FIG. 2.

If the suspension is sprayed under such a high pressure against thefilter discs that there might be a risk of the generated fine fractionhitting the opposite filter discs, a protecting circular partition wallmay be arranged between the filter discs of each pair of filter discs.Such a partition wall 28 extends between and parallel to the filterdiscs 4 and 5 in FIGS. 1 and 2.

The fractionation devices according to FIGS. 1-10 described above have asimple construction and are preferably operated with the same pressureon the coarse fraction sides and the fine fraction sides of the filterdiscs. Alternatively, however, it is quite possible to seal the coarsefraction sides from the fine fraction sides to enable operation of thefractionation device with a pressure difference between the coarse andfine fraction sides, whereby the capacity of the fractionation devicecan be increased. In such a case a source of overpressure may beconnected to an inlet 29 in the drain pipe 19 for coarse fraction and/ora source of underpressure may be connected to an inlet 30 in the drainpipe 17 for fine fraction. A drawback with this alternative, however, isthat each filter disc would need to be sealed against the housing 1 bymeans of some kind of sealing arrangement, which would make thefractionation device more expensive and increase the risk of breakdownsbecause of damaged seals.

The fractionation device shown in FIG. 1 comprises a housing 31, inwhich three pairs of annular filter discs 32 are attached parallel toeach other and to twelve horizontal drain pipes 33 for fine fraction.The latter are arranged concentrically and evenly distributed around thefilter discs 32 and form a drum shaft 34, which is rotatably journalledin the housing 31 and coupled to a drive motor 35. The filter discs 32are united with each other in pairs by means of walls 36 and 37, so thata space 38 for fine fraction is formed between the filter discs of eachpair.

The space 38 for fine fraction of each pair of filter discs 32 isdivided into twelve cells 39, which are evenly distributed around thecircumference of the filter discs. The twelve cells 39 communicate withtheir respective twelve drain pipes 33. The arrangement of the cells 39prevents that a too large an amount of fine fraction, which is in thespaces 38, overloading one or more of the drain pipes 33 whichmomentarily are the lowest during the rotation of the drum shaft. Thisavoids giving the fine fraction time to fill the fine fraction space 38,which would worsen the fractionation result, since the coarse fractionwould have a larger concentration of fine particles. However, it wouldbe possible to design the filter discs without cells if the flow ofliquid through the fractionation plant is kept low, such that thecapacity of a single drain pipe 33 is sufficient to continuously emptythe fine fraction space 38 of generated fine fraction. In the finefraction spaces 38 there are annular partition walls 55 arrangedparallel to the filter discs 32 to prevent the created fine fraction atone of the filter discs 32 of a pair of filter discs from disturbing thefractionation at the opposite filter disc.

The drum shaft 34 is provided with a gable 39a, which via an annulargasket 40 between the gable 39a and the housing 31 separates theinterior of the housing into a coarse fraction chamber 41, in which thefilter discs 32 are situated, and a fine fraction chamber 42, into whichthe drain pipes 33 open. The annular gasket 40 is attached to thehousing 31 and seals against an annular rail 43, which surrounds thedrain pipes 33 and is attached to these pipes (FIG. 13). In the finefraction chamber 42 there is an inlet 44 for connection to a source ofsub-pressure and in the coarse fraction chamber 41 there is an inlet 45for connection to a source of overpressure. The coarse and fine fractionchambers 41, 42 have bottom outlets 46 and 47, respectively.

A plurality of spray means 48 for the suspension are arranged to spraythe outer sides of the filter discs 32 of the three pairs of filterdiscs. At each filter disc 32 a number of the spray means 48 arearranged on seven radial pipes 49 (FIG. 12). The pipes 49 are connectedto a stationary distribution pipe 50 for supplying suspension extendingthrough the central holes of the annular filter discs 32.

The distribution pipe 50 at one of its ends is provided with an outlet51 in the coarse fraction chamber 41 for avoiding sedimentation ofparticles in the distribution pipe 50. A stationary distribution pipe 52for supplying cleansing liquid extends through the central holes of theannular filter discs 32. From the distribution pipe 52, pipes 53 extendvertically upwardly along the coarse fraction sides of the filter discs.The pipes 53 are provided with stationary spray means 54, which aredirected towards the filter discs 32.

The fractionation plant shown in FIG. 14 is an assembly of twofractionation devices of the kind shown in FIG. 11. Here, thefractionation takes place in two stages. For instance, the suspensionmay first be fractionated by means of one of the fractionation devicesin a first stage I, whereafter the obtained coarse fraction in stage Iis fractionated by means of the second fractionation device in a secondstage II. By this technique the coarse fraction obtained in stage IIwill have a relatively low concentration of fine particles.

The fractionation devices according to stage I and II have common drainpipes 56 for fine fraction, which form a drum shaft, and a commondistribution pipe 57 for supplying cleansing liquid. Two annularpartition walls 58 and 59 between the drain pipes 56 define a drainagechamber 60 for fine fraction from the second stage II. The drainagechamber 60 is provided with a bottom outlet 61 and communicates with theinterior of the drain pipes 56 via holes 62 in the drain pipes 56 (FIG.15). The drain pipes 56 are provided with partition walls 63, whichprevent communication between the interior of the drain pipes 56 instage I and stage II. A distribution pipe 64 for supplying suspension tostage II surrounds coaxially a distribution pipe 65 for supplyingsuspension to stage I.

If the fractionation plant according to FIG. 14 is desired to beutilized to produce a coarse fraction of a relatively low concentrationof fine particles, coarse fraction is pumped from a coarse fractionoutlet 66 in stage I to an inlet 67 in the distribution pipe 64 in stageII.

The fractionation plant shown in FIG. 16 is an assembly of twofractionation plants of the kind shown in FIG. 14, the fractionationtaking place in four stages I-IV. Analogous to the fractionation devicesaccording to FIG. 13 the fractionation devices according to stages I-IVin FIG. 15 have common drain pipes 68 for fine fraction and a commondistribution pipe 69 for supplying cleansing liquid. The fractionationdevices according to the stages II and III also have a commondistribution pipe 70 for supplying suspension. A partition wall 71 inthe distribution pipe 70 prevents communication between the interior ofthe distribution pipe 70 in stages II and III. Distribution pipes 72 and73 for supplying suspension to the stages I and IV, respectively, arearranged opposite to each other at the ends of the distribution pipe 70and surround the distribution pipe 70 coaxially. A drive motor 74, bymeans of chains or belts 75, is connected to the drum shaft formed bythe drain pipes 68 for rotation of this drum shaft.

In stage III there is shown tow filter discs 76 and 77, the surfaces ofwhich converge slightly towards each other in direction towards thecentre of the filter discs 76, 77, i.e. the axial distance A between theradial outermost edges of the filter discs 76, 77 is larger than thedistance B between the radially innermost edges of the filter discs 76,77. Thus the two filter discs 76, 77 take the shape of an hour-glass. Bythis shape a more uniform distribution of the flow of the coarsefraction along the filter discs is achieved, than by parallel surfacesof the filter discs, which by tests has proved to be advantageous incertain applications.

In stage IV there is shown a pair of discs 78 and 79, only one 78 ofwhich is provided with screen holes, while the other 79 is completelytight. Such a pair of discs 78, 79 may be utilized as needed for meetingthe fractionation capacity required. For instance, stage IV may includefour pairs of filter discs and one such pair of discs 78, 79.

FIGS. 14 and 16 there is shown for reasons of simplicity only one pairof discs in each fractionation stage. Of course, each stage may includean optional number of pair of discs, for instance five in a first stage,four in a second stage, three in a third stage and two in a fourthstage.

The fractionation plants according to FIGS. 14 and 16 may, for instance,be utilized for separating the valuable fibres (coarse particles) fromprinting ink (fine particles) when producing paper pulp from wastepaper. In this case, for instance, an efficiency of 60% may be obtainedin a first fractionation stage, i.e. there is 40% of the printing inkleft in the fibre suspension (coarse fraction), which leaves the firststage. Then, this fibre suspension may be fractionated in a secondstage, whereby perhaps there is 20% of the printing ink left in thefibre suspension leaving the second stage. In this way the fibresuspension may be cleaned from printing ink in further stages, so thatthe fibre suspension finally achieves a satisfactorily low concentrationof printing ink.

The fractionation plants according to FIGS. 14 and 16 may also beutilized for fractionating various media and/or be equipped with varioussizes of the screen holes in the respective fractionation stages forproducing fractions with various sizes of particles, since thefractionation stages can be operated independently of each other.

Alternatively, a plurality of fractionation devices of the kind shown inFIGS. 1 or 2 may be assembled in several fractionation stages in thesame way as described above for the fractionation device according toFIG. 11.

In FIG. 17 there is shown a filter disc divided into cells, which isattached to a tubular central shaft 80, around which twelve axial drainpipes 81 for fine fraction are centrally arranged in the filter disc.The interior of the twelve pipes 81 communicate with twelve cells 82,respectively, in the filter disc. Each cell 82 has straight wallportions 83 extending in direction radially outwards and forwards in therotation direction of the filter disc. A distribution pipe 84 forsupplying suspension extends parallel to the shaft 80 and is situatedradially outside the filter disc. A plurality of spray means 85 areconnected to the distribution pipe 84 and are arranged for sprayingabout a quadrant of one side surface of the filter disc. A distributionpipe 86 for cleansing liquid is provided with spray means 87 arranged tospray the same side of the filter disc as the spray means 85, and issituated above the shaft 80.

If the filter disc according to FIG. 17 is utilized in severalfractionation stages two adjacent stages are separated by means ofpartition walls 88 (FIG. 18) analogous to the above described inconnection with the fractionation plants according to FIGS. 14 and 16.The interior of the pipes 81 in one of said adjacent stages communicatesvia holes 89 in the pipes 81 with a drainage chamber 90 for finefraction situated between the partition walls 88.

An advantage with a fractionation device equipped with the filter discsaccording to FIG. 17 is that in a dismantled state it requiresconsiderably less space during transportation than a fractionationdevice according to FIG. 11, since the shaft 80 including the drainpipes 81 has a relatively small diameter and the cells 82 of the filterdiscs can be arranged detachable from the shaft 80. A drawback with thefilter discs according to FIG. 17, however, is that their fractionationcapacity is relatively low, since during operation only about 25% of thesurface of each filter disc can be sprayed with suspension. (If a largerpart of the surface of each filter disc is sprayed with suspension thecreated fine fraction in certain cells 82 cannot flow out into thecentral drain pipe 81 during a part of the revolution of the filterdiscs).

The fractionation device shown in FIGS. 20-22 in principle is of thesame kind as the fractionation device according to FIG. 12, but has afilter disc with only six cells 91. The cells 91 have radially directedwalls 92 and are connected to six drain pipes 93, respectively, for finefraction. Each drain pipe 93 is connected to two adjacent cells 91. Acentral distribution pipe 94 for supplying suspension is connected to aplurality of spray means 95, which during operation are arranged tospray a larger surface of the part of the filter disc which is movingdownwardly than the part moving upwardly. A turning movement thereforearises from the fine fraction in the cells 91, which results in thefilter disc becoming self-rotating, so that a separate drive motor forturning the filter disc is not needed. In certain upper turningpositions of the cells 91 the fine fraction can pass the cells 91downwardly in the filter disc via an annular channel 96 situated betweenthe distribution pipe 94 and the cells 91.

The spray means 95 are arranged on three stationary pipes 97-99extending in the circumferential direction of the filter disc atdifferent radial distances from the shaft 94. To prevent created coarsefraction flowing downwardly over the filter surface which is sprayedwith suspension and there disturbing the fractionation, two stationaryguide means in the form of chutes 100, 101 are arranged halfway betweenthe intermediate pipe 98 and the radially innermost pipe 97, and halfwaybetween the intermediate pipe 98 and the radially outermost pipe 99,(FIGS. 21 and 21). In the lowest portion of the chutes 100, 101 thereare outlets 102 and 103 for coarse fraction collected by the chutes 100,101. Two annular circular guide means 104 and 105 are attached to thecoarse fraction side of the filter disc coaxially with the filter discand extend axially a distance across the respective chutes 100, 101(FIG. 22). The guide means 104, 105 aim at conducting coarse fraction,which flows downwardly over the surface of the filter disc, to thechutes 100, 101.

To reduce the risk of generated fine fraction in the cells 91 flowingbackwardly out through the screen holes, an annular chute 106 isarranged along the circumference of the filter disc radially outside thecells 91. The chute 106 conducts the fine fraction to the drain pipes93, which currently are lowest during the rotation of the filter disc.

In FIGS. 23-26 there is shown an alternative arrangement of the spraymeans 107 for suspension intended for a fractionation device of the kindshown in FIGS. 1 and 2. The spray means 107 are arranged on four curvedpipes 108 connected to four stationary distribution pipes 109,respectively. From the centre of the filter disc each pipe 108 curvesoutwardly and forwardly relative to the rotational direction of thefilter disc. On each pipe 108 a plurality of the spray means 107 isarranged one after the other and directed towards the surface of thefilter disc, so that during operation the filter disc is hit bysuspension on an elongated area, which like each pipe 108 curvesoutwards from the centre of the filter disc and forwards relative to therotational direction of the filter disc. This means that duringoperation the suspension, which is sprayed against the filter disc,forms a curved wave on said elongated area, which creates a radiallyoutwardly directed component of movement in the flow of the coarsefraction on the surface of the filter disc.

Alternatively, each curved pipe 108 may be provided with a single spraymeans 110 designed with a long and narrow discharge opening forsuspension, which curves along the pipe 108 (FIGS. 25 and 26).

The arrangement described above for spraying a filter disc, such that itis hit by suspension over elongated curved areas, is not limited to thekind of fractionation device shown in FIGS. 1 and 2, but may also beutilized of a modified design in the kind of fractionation devices shownin FIGS. 11-22.

The spray means for suspension can be arranged to spray the filter discwith a jet, which is directed obliquely against the surface of thefilter disc (FIGS. 24 and 26). By adjusting the angle of the jet againstthe filter disc the sizes of the particles in the fine and coarsefractions can be controlled within certain limits, since the smaller theangle of the jet against the filter surface, the smaller the particleswhich can pass through the screen holes. Suitably, said obliquelyadjusted jet is directed backwards relative to the rotational directionof the filter disc.

The screen cloth 111 shown in FIG. 27 has a circular annular shape andis provided with a stretching ring 112, which extends through a tubularchannel formed by the screen cloth 111 at its radially outermost edgeand which keeps the screen cloth 111 stretched. The screen cloth 111 hastwo opposite edges, which extend from the centre hole of the screencloth to the outer periphery of the screen cloth and which are fastenedtogether by means of a fastening means, for instance a chainpull 113.The stretching ring 112 has two ends 114, 115 situated in front of thechainpull 113, which ends 114, 115 are joined by means of a sleeve 116.Alternatively, the stretching ring 112 may consist of several separateparts to facilitate an axial separation of said two opposite edges ofthe screen cloth 111.

The screen cloth 111 according to FIG. 27 is preferably utilized in afractionation device of the kind shown in FIGS. 1 and 2 and which thushas a centre shaft 117, to which the filter discs are attached, but mayalso be utilized in fractionation devices of the kind shown in FIGS.11-26. Each filter disc has a framework consisting of an inner annularbar 118 and an outer annular bar 119, which are joined to each other bymeans of four spokes 120 (FIG. 31).

When mounting the screen cloth 111 on the filter disc framework shown inFIG. 31, the chainpull 113 is kept in an open position at the same timeas the ends 114, 115 of the stretching ring are separated, so that thescreen cloth 111 can be applied on the shaft 117, after which thechainpull 113 is closed and the ends 114, 115 are joined by means of thesleeve 116. The screen cloth 111 is further stretched by pulling athread 121 extending through a channel formed by the screen cloth 111 atits radially innermost edge, whereafter the ends of the thread 121 aretied together.

Alternatively, the ends of the thread 121 may be fastened to the screencloth 111 in the vicinity of the chainpull 113 and at mutual sides ofthis, the screen cloth 111 being stretched by the thread 121 when thechainpull 113 is brought to its closed position. In this case thechainpull 113 should be arranged, such that it closes when pulledtowards the centre of the screen cloth 111. In addition, the thread 121may suitably be elastic or be constituted by a coil spring.

The stretched screen cloth 111 is clamped between the inner bar 118 andan inner clamping ring 122 by means of screw joints 123 and between theouter bar 119 and an outer clamping ring 124 by means of screw joints125. The clamping rings 122, 124 may be provided with annularprotrusions, which cooperate with annular grooves in the bars 118, 119,so that the screen cloth 111 is securely fastened between the clampingrings 122, 124 and the bars 118, 119 (FIGS. 32 and 33). To enablerestretching of the clamped screen cloth 111 the outer bar 119 can beprovided with a relatively deep annular groove 126, which cooperateswith an adjustment ring 127. The depth of penetration of ring 127 intothe groove 126 is adjustable by means of adjustment screws 128 (FIG.33). With the screen cloth 111 extending between the bar 119 and theadjustment ring 127 the screen cloth 111 thus can be further stretchedby turning the adjustment screws 128.

All of the components described above of the arrangement according toFIGS. 27-33 for providing the stretching of the screen cloth 111 areonly required in those cases where the screen cloth 111 is very thin andflexible. Thus, if the screen cloth 111 is relatively stiff the thread121 would not be needed. In certain cases neither would the adjustmentring 127. Should the screen cloth 111 be very stiff neither the thread121 not the clamping ring 112 would be needed and often not theadjustment ring 127. The screen cloth 111 may also be constituted by aplastic material shrinkable by heat, so that the screen cloth 111 can bestretched in a clamped position by heat treatment.

We claim:
 1. A device for fractionating suspensions, such as fiber pulp,comprising:a housing, a substantially horizontal shaft extending in thehousing and rotatably journaled in the housing, at least one pair ofsubstantially vertical filter sheets having screen holes and attached tothe shaft substantially transverse to the shaft, spray means adapted tospray suspension against the filter sheets, to separate the suspensioninto a fine fraction, which passes through the screen holes of thefilter sheets and which contains fine particles, and a coarse fraction,which does not pass through said screen holes and which contains coarseparticles and some fine particles, said spray means being positionedrelative to the filter sheets, such that one of said fine and coarsefractions is formed between the filter sheets of said pair of filtersheets, while the other fraction is formed outside said pair of filtersheets, additional spray means for spraying cleansing liquid against thefilter sheets, for cleansing the screen holes of the filter sheets,means for displacing the filter sheets relative to said additional spraymeans, such that said additional spray means spray cleansing liquidagainst the screen holes which have been sprayed with suspension, firstoutlet means for discharging the fine fraction from the housing, andsecond outlet means for discharging the coarse fraction from thehousing.
 2. A device according to claim 1, wherein the spray means forsuspension are positioned to spray the outer sides of said pair offilter sheets.
 3. A device according to claim 2, wherein a partitionwall extends between and substantially parallel to the filter sheets ofsaid pair of filter sheets.
 4. A device according to claim 1, where eachfilter sheet is substantially circular and arranged coaxially with theshaft.
 5. A device according to claim 1, wherein each filter sheet issubstantially circular and arranged coaxially with the shaft and whereinsaid spray means for suspension and said additional spray means forcleansing liquid are connected to a first distribution pipe and a seconddistribution pipe, respectively, said distribution pipes extending inparallel with said shaft.
 6. A device according to claim 5, wherein eachfilter sheet extends radially outwardly from the shaft.
 7. A deviceaccording to claim 5, wherein each filter sheet extends radiallyoutwardly from the shaft and the distribution pipes extend radiallyoutside each filter sheet.
 8. A device according to claim 7, wherein thehousing has a bottom space, in which during operation coarse fractionand fine fraction are collected, and comprising a plurality of partitionwalls for separating coarse fraction from fine fraction extending fromthe housing in the bottom space of the housing to the filter sheets andforming drainage chambers for fine fraction and coarse fraction situatedbetween and axially outside two adjacent partition walls at said pair offilter sheets.
 9. A device according to claim 8, wherein said means fordisplacing the filter sheets comprises a plurality of buckets arrangedon the fine fraction side of at least one filter sheet, such that duringoperation the buckets moving downwardly receive and collect finefraction, while the buckets moving upwardly are emptied of finefraction.
 10. A device according to claim 9, wherein the additionalspray means for cleansing liquid are arranged to spray upon the finefraction side of the filter sheets.
 11. A device according to claim 7,wherein the two filter sheets of said pair of filter sheets are unitedby means of wall portions, such that a space for fine fraction is formedbetween said two filter sheets.
 12. A device according to claim 11,wherein said space for fine fraction is divided into a plurality ofcells, which are evenly distributed around the circumference of thefilter sheets, and comprising a plurality of axial drain pipes for finefraction, the number of which is equal to the number of cells, saiddrain pipes extending concentrically around the shaft and being situatedradially inside the cells, the cells communicating with the interior ofthe drain pipes.
 13. A device according to claim 5, where the shaftforms a hollow drum, the filter sheets being annular and extendingwithin said drum.
 14. A device according to claim 13, wherein the twofilter sheets of said pair of filter sheets are united by means of wallportions, such that a space for fine fraction is formed between said twofilter sheets.
 15. A device according to claim 14, wherein a pluralityof axial drain pipes for fine fraction extend concentrically around saiddrum shaft and are situated radially outside each filter sheet, saidspace for fine fraction communicating with the interior of the axialdrain pipes.
 16. A device according to claim 15, wherein said drum isprovided with a gable and comprising an annular gasket between the gableand the housing which divides the interior of the housing into a coarsefraction chamber, in which the filter sheets are situated, and a finefraction chamber, into which each axial drain pipe opens.
 17. A deviceaccording to claim 16, wherein said space for fine fraction is dividedinto a plurality of cells of the same number as that of the drain pipesfor fine fraction, said cells being evenly distributed around thecircumference of the filter sheets and communicating with the interiorof the respective drain pipes.
 18. A device according to claim 17,wherein the interior of each drain pipe communicates with two adjacentcells of said pair of filter sheets.
 19. A device according to claim 18,wherein the spray means for suspension are arranged to spray, duringoperation, a larger surface of the part of the filter sheets which movesdownwardly than the part of the filter sheets which moves upwardly. 20.A device for fractionating suspensions comprising:a housing, arotatable, substantially horizontal shaft extending within the housing,a plurality of substantially vertical filter sheets attachedsubstantially transverse to the shaft, suspension spray means, placedbetween two filter sheets along the shaft, adapted to spray suspensionagainst the surfaces of the filter sheets facing the suspension spraymeans, such that a coarse fraction develops on the side of the filtersheets against which suspension is sprayed, and a fine fraction developson the side of the filter sheets against which suspension is notsprayed, cleansing spray means adapted to spray a cleansing liquidagainst at least one surface of the filter sheets, to cleanse the coarsefraction from the filter sheets, outlets into which the coarse and finefractions separately are drained from the housing.
 21. The deviceaccording to claim 20 and comprising means for displacing said filtersheets and said cleansing spray means relative to one another so thatsaid cleansing spray means sprays cleansing liquid against screen holeswhich have been sprayed with suspension.